Limited number of studies that directly measure reactive species or oxidative stress in patients with ARDS are presently. A major limitation for measuring reactive species is their short half life in biological systems. Since reactive species modify biological molecules such as proteins, lipids and DNA, measurement of the modified targets provide the experimental tools for their detection and quantification. Protein carbonyls are derived by the direct oxidation of amino acid residues or conjugation of aldehydes that are formed by the oxidation of unsaturated lipids or sugars. Overall plasma protein carbonyls indicate the formation of oxidants. Nitration of protein tyrosine residues results in the formation of 3-nitrotyrosine. Previous we found that the reaction of peroxynitrate with C02 provides the necessary nitrating agent that explains the formation of plasm protein 3-nitrotyrosine. Peroxynitrate is formed by the nearly diffusion limited reaction of nitric oxide and superoxide. Urinary isoprostanes are generated by reactive species attack on arachidonate in lipid bilayers and is selective and sensitive indicator of lipid peroxidation. Therefore, the main function of the analytical chemistry core (Core C) is to measure the levels of modified plasma proteins (carbonyls and 3-nitrotyrosine) and urinary isoprostanes in the human samples for Project 3. Protein carbonyls will be also measured in cell lysates, lung tissue, perfusate and plasma in samples generated in Project 4. Core C will also function as a central processing, storage and distribution facility for the samples collected in Project 3. The analytical chemistry will be located at the Institute for Environmental Medicine. Strengths of Core C include the established protocols for clinical specimen collection, processing and storage, the experience of the investigators with measuring these biological markers and the understanding of the biochemical origin of these biological markers.